Clock synthesis circuits are used to generate clock signals. Typically, the clock signals provide timing for operation of a circuit. In some applications, multiple timing references or clocks, which operate at different frequencies, are required. For example, some communication standards require operation of transmitter and receiver circuits at pre-determined clock frequencies. If a circuit supports multiple timing references, then multiple clock synthesis circuits are used.
Some circuit applications require a variable frequency clock. In general, a variable frequency clock is a clock that may change its frequency over time or for different purposes. There are many reasons why it is important to be able to vary the clock frequency that is used for timing in a computing system. For example, it is often important to vary the clock frequency in a computer system because of an interface between different controllers and components in the system.
A complicated circuit may require many different variances to the clock frequency for different portions of the circuit. The different required clock frequencies may span a very large and disparate range of different frequencies. In addition, the different clock frequencies may need to be directed to many different parts of the circuit.